Two oil tankers loaded with Saudi oil for China and India turned around in the Red Sea and headed toward the Suez Canal after a warning from Yemen’s Houthis, Al Jazeera reports.

On Monday, the Houthis announced a maritime blockade of Saudi Arabia. This creates a potential new front against the United States in its war with Iran, while also heightening the threat to global energy supplies and trade beyond the Persian Gulf.

In emails sent to shipping companies, the group urged them not to load or unload cargo in Saudi Arabian ports, warning that such actions could lead to strikes “at any point.”

The Houthis control northern Yemen, including the coast of the Bab el-Mandeb Strait at the entrance to the Red Sea. Saudi Arabia’s Red Sea port of Yanbu has served as the main alternative route for millions of barrels of Middle Eastern oil shipments, bypassing the blocked Strait of Hormuz at the outlet of the Persian Gulf.